sysutils/firstboot-pkg-upgrade: Ignore OSVERSION

Without IGNORE_OSVERSION=yes, images built from HEAD or stable branches
will hang during boot waiting for the user to agree to install updates
every time OSVERSION gets bumped.

This is a no-op for releases, since OSVERSION never changes there.

Reviewed by: ziaee
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D59053